Intracellular calcium may be the agent which adjusts the passive cation permeabilities of the apical and basolateral membranes during changes in the rate of transepithelial sodium transport in tight epithelia. these adjustments are critical for the cell to maintain a constant cellular ion content and volume during wide swings in the rate of transepithelial ion transport. Although previous experiments have demonstrated that the machinery for calcium to exert its effect in the cell is present, the relationship between the intracellular free calcium concentration (Ca ion) and transepithelial ion transport has not been examined. We have developed methods to measure simultaneously (Ca ion) and sodium transport in intact epithelia using calcium-sensitive fluorescent indicators, fluorescence microscopy and a silicon intensifier target camera. We will examine the role of intracellular calcium in regulating sodium transport in urinary epithelia. The goals of the project are: 1. To examine the relationship between (Ca ion) and sodium transport in tight epithelia. We will simultaneously measure both parameters as we vary the rate of transport by changing mucosal (Na) or adding the natriferic hormones vasopressin and aldosterone. 2. To assess the ubiquity of calcium's role in regulating ion transport in epithelia. We will measure (Ca ion) and sodium transport in MDCK and LLC-PK1 cells, both hormonally responsive and possessing sodium-coupled transport processes. 3. To identify the location and nature of the mechanisms by which (Ca ion) changes in relation to the rate of ion transport. We will first determine the source of calcium, either extracellular or intracellular, using digital image processing and inhibitors of calcium transport processes. We will identify the mechanism by which (Ca ion) increases, looking at sodium-calcium exchange, calcium channels and release from intracellular stores. We will identify the signal which triggers the increase in calcium, examining Na, (H+) and IP3 as possible mediators.